1. Field of the Invention
The present invention relates to a screw-type dry vacuum pump in which a rotor having a screw thread is disposed in a casing and rotated to discharge a gas from the pump.
2. Description of the Related Art
A conventional screw-type dry vacuum pump comprises, for example, two rotors each having a screw thread thereon, and slight gaps are formed between the two rotors and between a casing and each of the rotors. The two rotors are synchronously rotated in opposite directions, respectively, to transport a gas along the screw threads on the rotors. This type of pump has widely been used for exhausting a gas from a chamber in a semiconductor fabrication apparatus, for example.
In many semiconductor fabrication processes, a semiconductor wafer such as a silicon wafer is processed with use of a reactant gas in a semiconductor fabrication apparatus, such as a chemical vapor deposition (CVD) apparatus, a dry etching apparatus, or a sputtering apparatus, in which a vacuum environment is produced. A variety of processes, such as chemical vapor deposition, are performed in a chamber of the semiconductor fabrication apparatus. In some cases, process gases used in these processes contain solid materials or components that tend to be solidified. A screw-type dry vacuum pump, in which a rotor having a screw thread thereon is rotated to transport a gas, is suitable for exhausting these process gases through the rotation of the rotors. Solid materials that are accumulated in the screw threads and between the rotor and the casing are raked out and transported toward the discharge end through the rotation of the rotor.
FIG. 4 is a cross-sectional view showing a conventional screw-type dry vacuum pump, which is one type of dual shaft positive-displacement fluid machinery. The conventional screw-type dry vacuum pump comprises a cylindrical casing 11 and a rotatable rotor 12 having a screw thread thereon. The rotor 12 is housed in the casing 11 in such a state that a slight gap is formed between the outer circumferential surface of the rotor 12 and the inner surface of the casing 11. The rotor 12 is connected to a main shaft 15, which is supported by a bearing 16 fixed to a fixed member 13 on the discharge end. The main shaft 15 is coupled to an external motor (not shown), and hence the rotor 12 is rotated by actuation of the motor. A gas is introduced into the pump through an inlet port 17 by the rotation of the rotor 12. The gas is transported toward the discharge end along the rotating screw thread and then discharged from an outlet port 18.
The reactant gas to be discharged may contain solid materials. Accordingly, when the screw-type dry vacuum pump is used for exhausting a chamber of a semiconductor fabrication apparatus, for example, components in the reactant gas are solidified and accumulated on the discharge end of the rotor as reaction by-products 20 in some cases. As described above, in a screw-type dry vacuum pump, a rotor having a screw thread thereon is rotated to transport a gas, and solid materials that are accumulated in the pump can be raked out and transported toward the discharge end through the rotation of the screw thread. However, the gap formed between the rotor 12 and the casing 11 is filled with the reaction by-products 20 near the discharge end in the casing 11, as shown in the FIG. 4. When solid materials are accumulated on the discharge end of the vacuum pump to fill the gap formed between the rotor and the casing, an excessive load is imposed on the motor to stop the pump, causing damage to the pump and the motor.
The reactant gas to be discharged may contain agglomerates of reaction by-products, foreign matter, and the like. When the particle diameters of these solid materials are smaller than a gap between the rotors or a clearance between the rotor and the casing, the solid materials will be discharged by the evacuating operation of the rotors, as described above. However, when the particle diameters of the solid materials are larger, the solid materials may be caught in the gap between the rotors or the gap between the rotor and the casing in the evacuating process, hindering the rotation of the rotors.
The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a vacuum pump which can prevent reaction by-products from being accumulated inside the pump.
According to a first aspect of the present invention, there is provided a screw-type dry vacuum pump comprising: a rotor having a screw thread thereon for exhausting a gas through rotation; a casing for housing the rotor therein; a reduced portion provided in the casing, the reduced portion forming a slight gap between the outer circumferential surface of the rotor and the inner surface thereof; and an enlarged portion provided on a discharge end of the casing, the enlarged portion forming a larger gap between the outer circumferential surface of the rotor and the inner surface thereof.
Since an enlarged portion, which forms a larger gap between the outer circumferential surface of the rotor and the inner surface thereof, is provided on a discharge end of the casing, a large gap is formed between the rotor and the casing on the discharge end. The present invention resolves the conventional problem that reaction by-products are accumulated in a fine gap between the rotor and the casing. Accordingly, the present invention ensures stable pump operations even when discharging a gas prone to generating solid reaction by-products.
According to a preferred aspect of the present invention, the vacuum pump further comprises an enlarged portion provided on a suction end of the casing, the enlarged portion forming a larger gap between the outer circumferential surface of the rotor and the inner surface thereof.
When the pump is provided with a plurality of rotors, a depression may be formed in the casing between the adjacent rotors to allow the accumulation of reaction byproducts.
According to another preferred aspect of the present invention, the vacuum pump further comprises a discharge mechanism for discharging reaction by-products that have been accumulated inside the casing. With this construction, reaction by-products can be discharged outside the pump when a large amount of by-products has been accumulated. Accordingly, this construction can prevent sudden stoppage of the vacuum pump and ensure stable operations of the pump.